Menu and Back button don't work?

I sold my phone on eBay, and shipped the phone out. When it got to the buyer, he stated that the back and menu button do not work. He also flashed a i9500 rom, which may or may not have helped.

I shipped it out under warranty but it says "Knox warranty void" in the summary of the repair. Anyone know how I can fix this problem myself? I tried flashing the NAE rom and while everything else worked, the buttons did not.

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